1. Field of Invention
This invention relates to a capacitance type sensing device which is responsive to displacement.
2. Description of Related Art
Many devices are known which use change in capacitance of a structure as displacement takes place in parts of that structure, in order to measure displacement. One such known device is disclosed in U.S. Pat. No. 5,006,952. That patent discloses a joystick which has spaced flat capacitance plates and a circuit for measuring the change in capacitance of those plates when relative displacement of the plates takes place. The plate arrangement consists of a single circular middle disc sandwiched between two others all separated by an air gap. Movement of the joystick causes the middle disc to move also. However the movement of the middle disc is limited to the width of the air gap. When the middle disc tips from side-to-side about its axis there is little change in the detected capacitance, whereas a relatively large change in capacitance takes place when the middle disc moves in the direction of its axis. Such a marked difference in the response to displacement in the two different directions causes problems because the joystick will respond with different capacitance changes when pushed in different directions.
Cylindrical capacitance type displacement measurement devices have been disclosed e.g. European Patent No. 0831300,U.S. Pat. No. 4,961,055 and U.S. Pat. No. 3,928,796. In each of these prior art disclosures the devices are intended to operate such that respective cylindrical parts move axially so that there is no change in the size of the dielectric between those parts.
The present invention according to a first aspect provides a displacement responsive device comprising a first member and a second member displaceable with at least three degrees of freedom with respect to the first member and comprising a capacitance displacement sensor having at least two capacitive elements in spaced relation, one of the elements being connected to the first member and the other of the elements being connected to the second member such that the two elements are displaceable with respect to each other in the said at least three degrees of freedom so as to vary their capacitance, the invention being characterised in that the elements are in the form of at least two substantially coaxial tubes one of which is disposed within the other.
Preferably the tubes are two generally concentric cylinders. Alternatively there are three tubes, a central tube and two others one inside the central tube and one outside the central tube.
The invention relates also to an arrangement of springs which form a suspension system for a displacement responsive device, e.g. a measurement probe.
Such an arrangement in a probe is known for example from International Publication No. WO00/60307, in which there is described a measuring probe having a stylus holder mounted for pivoting motion and axial translation on a pair of axially separated planar springs which extend between the stylus holder and the probe housing. The stylus holder pivots about a point on the probe axis, and which is positioned somewhere between the planes of the two springs depending on the relative stiffnesses of the two springs. Additionally it is suggested in this publication that a third spring could be added which also extends between the stylus holder and the housing in a plane through the pivot point of the stylus holder.
Thus according to a second aspect of the invention there is provided a suspension system for a displacement responsive device in which a stylus holder is suspended within the housing by means of an alternative arrangement of springs.
In a preferred embodiment of the invention the springs are planar springs.
Also in a preferred embodiment of the invention the suspension system comprises three planar springs.
Preferably the probe additionally includes an overtravel device, which may be a kinematic overtravel device known per se, to protect the spring suspension against overstressing of the springs during large movements of the stylus in a probing operation.
The overtravel mechanism is preferably disposed within the spring suspension system.